The present invention relates generally to digital imaging systems. Particularly, the present invention relates to an edge and pattern recognition technique for x-ray to light field alignment in digital radiographic image systems.
Digital x-ray imaging systems are becoming increasingly widespread for producing digital data, which can be reconstructed into useful images. In current digital x-ray imaging systems, radiation from an x-ray source is directed toward a subject, typically a patient in a medical diagnostic application. A portion of the radiation passes through the patient and impacts a detector. The surface of the detector converts the radiation to light photons, which are sensed. The detector is divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. Because the radiation intensity is altered as the radiation passes through the patient, the images reconstructed based upon the output signals provide a projection of the patient""s tissues similar to those available through conventional photographic film techniques.
In available digital detectors, the surface of the detector is divided into a matrix of picture elements or pixels, with rows and columns of pixels being organized adjacent to one another to form the overall image area. When the detector is exposed to radiation, photons impact a scintillator coextensive with the image area. A series of detector elements are formed at row and column crossing points, each crossing point corresponding to a pixel making up the image matrix. In one type of detector, each element consists of a photodiode and a thin film transistor. The cathode of the diode is connected to the source of the transistor, and the anodes of all diodes are connected to a negative bias voltage. The gates of the transistors in a row are connected together and the row electrode is connected to scanning electronics. The drains of the transistors in each column are connected together and each column electrode is connected to additional readout electronics. Sequential scanning of the rows and columns permits the system to acquire the entire array or matrix of signals for subsequent signal processing and display.
In use, the signals generated at the pixel locations of the detector are sampled and digitized. The digital values are transmitted to processing circuitry where they are filtered, scaled, and further processed to produce an image data set. The image data set may then be used to store the resulting image, to display the image, such as on a computer monitor, to transfer the image to conventional photographic film, and so forth. In the medical imaging field, such images are used by attending physicians and radiologists in evaluating the physical conditions of a patient and diagnosing disease and trauma.
The installation and setup procedures for digital imaging systems, such as radiographic diagnostic imaging systems, can be complex and time-consuming. For example, to comply with customer image quality and consistency requirements and various regulatory and safety standards for diagnostic imaging systems, such procedures generally require the determination of a variety of factors, including the accurate positioning of the x-ray source with respect to the x-ray detector. Additionally, the determination of the separation distance between the x-ray source and x-ray detector, referred to as the source-to-image distance (SID), must also be established. Moreover, the setup generally requires that the x-ray field produced by the source be accurately positioned to avoid excessive exposure to radiation and the possible need to retake desired exposures.
To minimize the administration of non-diagnostic radiation to human patients, it is desirable to control and limit x-ray exposure that is unnecessary for creating the image data set. Hospitals typically control and limit x-ray exposure by conforming to regulatory standards. For instance, on radiographic systems, a visible light beam is often used by the operator to position the diagnostic x-ray source assembly with respect to the patient. Regulatory standards limit allowable misalignment between the projected visible light and radiation fields to ensure delivery of x-rays to the desired area. These standards generally restrict the total misalignment of the four edges of the projected rectangular field to a stated percentage of the source to image distance (SID).
United States Department of Health and Human Services criteria restrict the misalignment of the light and x-ray beams to be less than 2% of the indicated SID for the system. Users, typically, perform testing and record the results for each system prior to turnover of the system for diagnostic use and periodically in accordance with a quality assurance processes thereafter during the lifecycle of the system.
In a conventional field alignment test, the edges of the light field are located using some type of visible/radio-opaque tool to mark the position. Next, an x-ray exposure is taken and the physical edges of the radiation field are measured in relationship to the corresponding light-field marks on the resulting image. This process is manual and inexact, and therefore is characterized by high statistical variance. Because of the error associated with the measurement process, it is necessary to set internal rejection limits significantly below the limits mandated by the regulatory agencies. In cases where systems exceed the rejection limit, it is necessary to readjust the diagnostic source assembly, reposition the collimator or light bulb, and then retest to confirm compliance with the centering criteria.
There is need, therefore, for a novel technique, for determining radiation field to light field decentering on digital radiographic image systems.
The invention provides a novel technique for x-ray field alignment designed to respond to such needs. The technique utilizes a radio-opaque template with unique geometric features and attributes that are subsequently projected onto a digital detector by means of radiographic imaging and subsequently analyzed in a manner that automates the computation of the x-ray to light field misalignment. The technique includes a method for calibrating an imaging system. The method includes positioning an x-ray source to generate an x-ray beam at a detector. The method also includes directing a light beam at the director where a light field is produced and provides the area which the radiographic template is aligned. The x-ray field is then compared to the aligned template and the distances between the peripheral edges of the radiation field and the template are calculated. Once the distances are computed to determine the location of the vertices of the radio-opaque pattern, the offset distance is computed. The radiation field is aligned with the light field, providing a predictable exposure area and reducing the amount of non-diagnostic radiation delivered to a patient.